Field of the Invention
The present invention relates to a miniature hearing aid having a multilayer circuit arrangement.
Description of the Invention
Multilayer circuit boards are often utilized in areas of technology, where the manufactured products are very small in size. The design of small electronic products usually calls for printed board assemblies which are themselves as small as possible, so as to use as little of the minimal space within the product as possible. In hearing aid technology, for example, circuit boards are constructed such that they, together with the components mounted thereon, require minimum space within the hearing aid housing. Furthermore, it is advantageous if flexible circuit boards are used, so that they can be bent to fit the irregular interior space of the housing.
A number of circuit boards have been developed which only partially fulfill the requirements described above. For example, German patent application No. 27 24 399 (corresponds to U.S. Pat. No. 4,037,047) describes a three to ten layer circuit board with connection appendages of limited flexibility. The circuit boards themselves, however, are not adapted for use in very small products. Their circuitry connections make inefficient use of space, in that they are formed by plated through holes. A standard practice in the industry for several years, the process of forming plated through holes encompasses puncturing holes through all the layers of a circuit board, even when electrical connections are to be established, for example, only between on outer layer and an interior layer. In such a case, the through hole opens onto the other outer layer, but cannot be used for further connections. Especially if components are mounted on the circuit board, the resulting through holes lead to an inefficient component distribution. The component density cannot be maximized.
Contraves AG, a Swiss Corporation, proposes a partial solution to this problem in a European patent application No. 0 183 936 and in their advertisement: Bulletin H102d8603CZ. The component density is increased by providing blind holes as well as through holes for electrically connecting the various layers of the circuit board. A maximum density can, however, still not be achieved, since interior layers are connected by drilling from the outside through at least one outer layer. This process has a further shortcoming, since boring blind holes in multilayer boards requires relatively thick layers. Even when working with utmost precision, the insulating sheets between the conductive layers must be sufficiently thick (approximately 125 .mu.m) to provide the necessary drilling tolerances. This, in turn, leads to non-flexible circuit boards, unsuited for hearing aids.
German patent application No. 1 160 519 and U.S. Pat. No. 3,436,819 also describe methods for manufacturing multilayer circuit boards. Circuit boards produced by these methods are, however, also rigid. A further circuit board with flexible connectors, described in U.S. Pat. No. 3,471,348, similar to the board described in U.S. Pat. No. 4,037,047, is also too rigid to be used in modern hearing aids.